Regarding its principal features, the stock feed at a paper machine is generally as follows. The stock components are stored at the paper mill in separate storage towers. From the storage towers, the stocks are fed into stock chests, and from the stock chests further into a common blend chest, in which the stock components are mixed with each other. From the blend chest, the stock is fed into a machine chest, and from the machine chest, there is an overflow back into the blend chest. From the machine chest, the stock, which is usually at a consistency of about 3%, is fed into a wire pit placed in the short circulation. In the wire pit, the high-consistency stock is diluted to a headbox consistency, which is usually about 1%.
The fibers and fillers which are used as the raw-material are passed onto a wire through the headbox while carried by water. The filtrate that has passed through the wire, which filtrate contains an abundance of fibrous material and fillers, is returned, as a diluting agent for the high-consistency stock coming from the machine chest, through the headbox back onto the wire. The flow loop thus formed is called the short circulation. The short circulation, together with the headbox connected with it, is commonly considered to be the most sensitive part of the papermaking process. Even little changes in the consistency, in the flow, or in other parameters have an immediate effect on the quality of the paper produced or cause web breaks in the paper machine.
Along with the high-consistency stock or along other paths, impurities may enter into the short circulation, which impurities must be removed before the headbox. This takes place by means of cleaning devices of the short circulation, which are, for example, centrifugal cleaners, screens and machine screens.
Ever stricter requirements of protection of the environment have resulted, in connection with paper and board machines, in more closed systems and also in a more closed short circulation and in as efficient recycling of raw-materials as possible. On the other hand, improved efficiency of production and minimizing of disturbance in the production are also aimed at. For this reason, among other things, a higher level of wire retention is used, which requires an increased use of retention agents.
The short circulations used in the present-day paper and board machines are rather complex, and the main line of the process includes an abundance of equipment, in which case, the process space required by the devices must be large. One reason for the complex nature of the short circulation of a paper or board machine is the binding of air in the circulation water in an open wire section. In order to remove the air from the water, it is necessary to construct one or even several deaeration systems. Air is bound in water in the wire section because the process portion after the wire is open and the circulation water is in direct contact with the surrounding air. Air is present in the circulation water both as air bubbles and in dissolved form. When the stock that is used for manufacture of paper is diluted with circulation water that contains air, the content of air in the water produces disturbance of many sorts in the formation of the paper web. Among other things, the content of air lowers the capacity, deteriorates the quality of the paper, and causes contamination of the process, formation of slime, blocking of cleaning devices, and wear.
Predicting vibrations in a system of short circulation is substantially more difficult than predicting purely mechanical vibrations. This results, among other things, from the fact that the coefficient of elasticity of flowing liquid also depends, to a great extent, on the air contained in the liquid. Also, the rigidity of the pipe systems and of the chests or tanks affects the rigidity of the system and, thus, the natural frequencies. Further, the velocity of progress of a pressure pulse in the stock slurry is slowed down substantially in compliance with the amount of undissolved air. Resilience of the walls in the pipe systems also has an effect slowing down the velocity of a pressure pulse. The variations arising from these factors have direct effects on the quality of the paper and are noticed as defects in the final product. Changes in the content of air in the stock also cause faults in the flow rate in the headbox. For example, air worsens the vibrations of the short circulation in the way mentioned above. Also, the air affects the density of the liquid to be pumped, and thereby it affects the pressure produced by a pump, and further it affects the basis weight.
For removal of air from the circulation water, a number of complex solutions are known in the prior art, which solutions involve additional devices and combinations of additional devices which result in additional costs of investment and operation, such as deaeration equipments, pumps, and chests or tanks. It is partly for this reason that the process volume of the main line becomes relatively large, as a result of which changes of paper grade in a paper machine require a long grade change time. Further, in the prior art processes, blend chests and stilling tanks have been used in order to keep the process conditions as invariable as possible.
In the current assignee's Finnish Laid-Open Publication No. 88,415, a process arrangement is described for production of headbox stock for a paper machine in the short circulation. In this arrangement, no fresh stock is mixed with the circulation water passing to the deaeration tank. In order to achieve this, in the arrangement, a combination wire pit is employed, which has been divided into two compartments or into two jointly operative tanks. The first tank is arranged as a feed tank for deaeration, and the second tank is arranged as a dilution tank for headbox stock, into which latter tank the fresh stock is fed. By means of this process arrangement, the principal objective has been to eliminate the essential problems produced by variations in consistency and by variations in pressure in the headbox.
On the other hand, in Finnish Laid-Open Publication No. 93,132 (in the name of Oy Tampella Ab), an integrated headbox and former arrangement is described, in which the stock is not in contact with the surrounding air as it is transferred from the headbox to the former. Also, the gap former used in the arrangement is closed, so that the stock and the white water cannot contact with the surrounding air. The draining of water in the former takes place by means of water drain boxes. For this integrated headbox-former unit, the designation CFF unit (Control Flow Former) is used.
In Finnish Laid-Open Publication No. 81,965 (also in the name of Oy Tampella Ab), a gap former is described in which the wires are supported on deck elements of closed box-like water drain spaces. The deck elements in the water drain space at the side of one of the wires are loaded resiliently against the wire in the desired way. Thus, in this former, pressure is used as an aid for the draining of the web, in which manner it is possible to enhance the separation of the solid matter from the suspension.